Viscosity Of Phase Research Articles

Polymer enhanced foam for improving oil recovery in oil-wet carbonate reservoirs: A proof of concept and insights into the polymer-surfactant interactions

The feasibility of polymer enhanced foam (PEF) for enhanced oil recovery (EOR) in a strongly oil-wet and heterogeneous carbonate reservoir with medium temperature (131 °F) and high salinity was experimentally investigated. An Alkyl Poly-Glycoside (APG) surfactant was firstly selected because of its prominent foam behavior in oil-wet carbonates. The effect of polymer type, flow rate and brine composition on foam strength and stability were systematically investigated, aiming to understand how the interactions between polymer and surfactant could influence the transport behavior and performance of PEF in porous media. It was found that, with either an associative polymer or a nonionic polymer (PAM), the foam strength and foam stability could be largely enhanced, both in the absence and in the presence of oil. A significant improvement in oil recovery efficiency was obtained for PEF with an associative polymer, approximately 5% higher than that achieved for conventional polymer-free foam. The PEF behavior is intimately dependent on the interactions between the polymer and the surfactant. It is hypothesized that the increased foam strength by adding polymer at oil-wet conditions may be resulted from an increase of aqueous phase viscosity, the wettability alteration from oil-wet to less oil-wet, the generation of a highly viscoelastic film and (or) the creation of a more stable pseudo-emulsion film. This study provides a robust approach for PEF mobility control and reservoir conformance in oil-wet and heterogeneous carbonate reservoir under analogous reservoir conditions.

Boiling heat transfer of CO2/lubricant on structured surfaces using molecular dynamics simulations

In this paper, the molecular dynamics (MD) simulations are employed to investigate the effects of lubricant (PEC4) on the boiling heat transfer of CO2 on the structured surface. The results show that the structured surfaces can enhance the nucleation boiling of CO2 by providing effective bubble nucleation sites and larger heat transfer areas. However, with the increase of the mass fraction of PEC4, the nucleation boiling heat transfer efficiency decreases on the structured surface. The distribution characteristics of the CO2/PEC4 mixture during the boiling process show that parts of CO2 molecules in the liquid mixture are more likely to be adsorbed by the double-bonded oxygen atoms and carbon atoms of the ester groups of the PEC4 molecules, which hinders the movement and phase transition process of CO2 molecules. Therefore, the PEC4 molecules in the nucleation region first migrate to the top liquid region before bubble nucleation, and then bubble nuclei comes into being. When the mass fraction of PEC4 is higher than 7 wt%, the nucleation temperature is increased by 10 K to 20 K. The decrease of the CO2 nucleation boiling heat transfer performance caused by PEC4 istheconsequenceof thecoaction ofmanyfactors, including that the lubricant leads to a decrease in self-diffusion coefficient of CO2, an increase in liquid phase viscosity, an increase in bubble nucleation temperature of CO2, and a decrease in heat transfer resistance of the solid-liquid interface.

Effects of Al2O3 content on the properties of BF slag and suitable fluxing regime for BF ironmaking with high Al2O3 content

ABSTRACT The difficulty of smelting high-alumina iron ore is that the Al2O3 content in the slag is too high, affecting the smooth running of the blast furnace. This work employed a combination of physical experiments and thermodynamic calculations to investigate the effect of Al2O3 content on blast furnace slag properties, including phase transition, viscosity, liquidus temperature, and slag structure. The results showed that a spinel phase appeared when the Al2O3 content was increased from 15wt% to 17wt%, and the liquidus temperature and viscosity of the BF slag were of therefore increased. For 17wt% to 20wt% Al2O3 content, the viscosity, liquidus temperature, and the ratio of complex structure content to simple structure content increased rapidly owing to slag structure polymerisation. Additionally, the viscosity and liquidus temperature of high-alumina blast furnace slag were negatively correlated with TiO2 content, but positively correlated with basicity. At approximately 5wt% TiO2 content, Ti(C, N) with a high melting point appeared in the molten iron, resulting in a rapid increase in the viscosity. Thus, it is recommended that the TiO2 content in the slag be less than 5wt% when the high-alumina iron ore is the main raw material of BF.

Modeling of counter-current spontaneous imbibition in independent capillaries with unequal diameters

Spontaneous imbibition is a crucial process for oil recovery from fractured and unconventional reservoirs. Herein, with the assumption of capillaries being independent, a new mathematical model for spontaneous imbibition is proposed and solved using a numerical method. The simulated results show that the wetting phase preferentially enters smaller capillaries where the advancement velocity is higher than that in larger ones, while the non-wetting phase can be displaced out in the larger capillaries. In addition, the effect of fluid viscosity ratio on counter-current imbibition is analyzed. The results show that imbibition velocity becomes higher with the increase in the viscosity ratio. When the viscosity of the non-wetting phase is larger than that of the wetting phase, the end pressure gradually increases as the imbibition front advances. In contrast, when the viscosity of the non-wetting phase is less than that of the wetting phase, the end pressure decreases with the infiltration. With a higher viscosity ratio of non-wetting and wetting phase, the heterogeneity of the interface advancement among different capillaries increases. Cited as: Chen, K., Xu, H., Zhang, Z., Meng, Q., Zhang, T. Modeling of counter-current spontaneous imbibition in independent capillaries with unequal diameters. Capillarity, 2022, 5(6): 115-122. https://doi.org/10.46690/capi.2022.06.02

The structure-function relationships of pectins separated from three citrus parts: Flavedo, albedo, and pomace

Citrus pectin is the main source of commercial pectin, which is widely used as an emulsifying and gelling agent. However, there is no information on the chemical structure of pectins from different citrus parts, and their functional properties are unclear. In this study, pectins from flavedo, albedo, and pomace were separated by a fractionated processing method, and their chemical structures and emulsifying and gelling properties were investigated to clarify the structure-function relationship. Interestingly, for both pomelo and grapefruit, pectins from different parts showed similar principles; that is, pomace pectin and albedo pectin showed the strongest emulsifying and gelling properties, respectively. Pomace pectin had a high degree of esterification (DE; 86.9%) and medium molecular weight (Mw; 177.7 kDa), and the most compact conformation (i.e., lowest Dmax and Rg values). Thus, it could form a thick layer at the oil-water interface and improve the viscosity of the continuous phase, thereby stabilizing the emulsion. Albedo pectin had the highest DE (89.8%), Mw (258.1 kDa), and proportion of HG region (87.7%); therefore, it gelled rapidly and had the tightest network structure. The findings provide insight into the structure-function relationship of pectin from different citrus parts and will facilitate targeted production of citrus pectin for high-value utilization.

Targeted modifications of citrus pectin to improve interfacial properties and the impact on emulsion stability

In the present work, three specific low weight average molecular weight (M w ) demethoxylated citrus pectin derived compounds (oligosaccharides) (CP 25 kDa , CP 3 kDa and CP 3.8 kDa ) were generated by means of enzymatic degradation and subsequent chemical demethylesterification of three starting pectin materials (CP74 144 kDa , CP53 151 kDa and CP38 116 kDa , respectively). These pectin oligosaccharides were structurally characterized, evaluated for their physicochemical properties, and their emulsifying and emulsion stabilizing (E F E S ) potential. In terms of structure, significant differences in M w , degree of methylesterification (DM), monosaccharide and protein contents were observed between the initial and the generated pectin-derived samples. Interestingly, upon evaluation of the physicochemical properties of the generated pectin-derived materials, it was observed they exhibited considerably improved interfacial tension properties in comparison to the initial pectin samples. Furthermore, all pectin materials were negatively charged to similar extents. On the negative side, the low viscosity of the generated pectin compounds was observed to significantly hinder their E F E S properties. Upon refrigerated storage of the pectin emulsions, it was observed that the initial pectin materials (at pH 2.5) exhibited better E F E S potential in comparison to the generated pectin materials, despite the superior interfacial potential of the latter samples. This was mainly ascribed to the higher viscosity of the non-degraded pectin samples, in turn attributed to their larger M w . Overall, the results obtained indicated that the M w and DM of the polymer played an important role in the interfacial tension properties of pectin. However, viscosity of the continuous phase of the dispersion is a critical factor in the E F E S potential of pectin. • All modified citrus pectin samples (M w <25 kDa) evaluated exhibited enhanced surface activity. • Coalescence and flocculation were considered the main destabilizing mechanisms in the emulsions formulated with low M w (<25 kDa) pectins. • Differences in viscosity increasing capacity, among the mother and modified pectin samples, are suggested to be the main factor influencing the stability of the formulated emulsions.

Dynamic wetting characteristics during droplet formation in a microfluidic T-junction

• Dynamic wetting characteristics during droplet formation in a microfluidic T-junction are investigated. • The three-phase contact line dynamics are influenced by the channel wettability and other flow properties. • The dynamic contact angle increases with an increase in Capillary number, static contact angle, and dispersed phase viscosity. • In contrast it decreases with an increase in flow rate ratio and slip length. Droplet formation in a microfluidic T-junction is dominated by the interfacial force arising between the two fluids. The three-phase contact line at the channel wall is affected by the surface energy or the wettability of the channel wall and other hydrodynamic forces. The dynamics of a three-phase contact line can be evaluated in terms of dynamic contact angle, which a moving contact line makes with a wall. The present work investigates the influence of wall hydrophobicity and fluid properties on the three-phase contact angle during droplet formation in a microfluidic T-junction. Experimental and numerical investigations are performed to study the effects of flow properties on the dynamic wetting of the channel during the droplet formation process. A level-set-based numerical model is employed to predict the moving contact line for different operating parameters and wettabilities of channel walls. Further, a simplified analytical model is developed to express the influence of flow parameters on the dynamic wetting behavior of the wall. The interface is more stressed out for a low viscous dispersed fluid, whereas the interface is less affected by a high viscous dispersed phase fluid. During the transient droplet stage, the dynamic contact angle increases with time with the increase in channel wall hydrophobicity. The dynamic contact angle increases with time for a higher static contact angle, and the variation decreases at a lower static contact angle. At the steady-state, the dynamic contact angle increases with the static contact angle and dispersed phase viscosity. In contrast, it decreases with an increase in the flow rate ratio and the slip length. The droplet with a higher length tends to increase the wetting line in the channel wall. The wettability of the channel wall influences the droplet interface along the lateral direction resulting in corner leakage flow, which further affects the dynamic contact angle.

Mixed burden softening-melting property optimization based on high-silica fluxed pellets

The development of a high proportion of pellets smelting process suitable for China's resource structure can promote low carbon and green development of blast furnace ironmaking. The differences in metallurgical properties, phase evolution, microstructure, and softening-melting characteristics between high-silica acid pellets and high-silica fluxed pellets were compared and analyzed. The high content of low-melting-point liquid phases such as Fe 2 SiO 4 within the high-silica acid pellets, leads to a low initial softening temperature, and a higher amount of quartz in the form of solid phase mass points will reduce the fluidity of the slag and makes the permeability index deteriorate. The high-silica fluxed pellets slag is mostly in the form of Ca 2 SiO 4 , softening-melting behavior, and the permeability of the material layer improving significantly. Using high basicity sinters with a high proportion of fluxed pellets, although not effective in reducing slag amount, was observed to have a synergistic optimizing effect on the softening-melting behavior of the mixed burden. As the proportion of fluxed pellets increases from 35% to 45%, T d decreases from 1455 °C to 1427 °C, and the melting zone narrows from 139 °C to 109 °C, S-value reduced from 1147.98 kPa·°C to 440.36 kPa·°C. With the increase in the proportion of high-silica fluxed pellets, the position of the cohesive zone moves up, the viscosity of the slag decreases, fluidity increases and the permeability of the material layer gradually increases. • Dendritic quartz increases acid pellets' slag phase viscosity and reduces fluidity. • The flux containing calcium optimizes the slag phase composition of the pellets. • The cohesive zone of the high-silica fluxed pellets becomes narrower and the permeability is significantly improved. • As the proportion of high-silica fluxed pellets increases, the permeability of mixed burden gradually improves.

Carbon Nanotube Migration in Melt-Compounded PEO/PE Blends and Its Impact on Electrical and Rheological Properties.

In this work, the effects of MWCNT concentration and mixing time on the migration of multi-walled carbon nanotubes (MWCNTs) within polyethylene oxide (PEO)/polyethylene (PE) blends are studied. Two-step mixing used to pre-localize MWCNTs within the PE phase and subsequently to observe their migration into the thermodynamically favored PEO phase. SEM micrographs show that many MWCNTs migrated into PEO. PEO/PE 40:60 polymer blend nanocomposites with 3 vol% MWCNTs mixed for short durations exhibited exceptional electromagnetic interference shielding effectiveness (EMI SE) and electrical conductivity (14.1 dB and 22.1 S/m, respectively), with properties dropping significantly at higher mixing times, suggesting the disruption of percolated MWCNT networks within the PE phase. PE grafted with maleic anhydride (PEMA) was introduced as a compatibilizer to arrest the migration of MWCNTs by creating a barrier at the PEO/PE interface. For the compatibilized system, EMI SE and electrical conductivity measurements showed a peak in electrical properties at 5 min of mixing (15.6 dB and 68.7 S/m), higher than those found for uncompatibilized systems. These improvements suggest that compatibilization can be effective at halting MWCNT migration. Although utilizing differences in thermodynamic affinity to draw MWCNTs toward the polymer/polymer interface of polymer blend systems can be an effective way to achieve interfacial localization, an excessively low viscosity of the destination phase may play a major role in reducing the entrapment of MWCNTs at the interface.

Identification and Mapping of Three Distinct Breakup Morphologies in the Turbulent Inertial Regime of Emulsification—Effect of Weber Number and Viscosity Ratio

Turbulent emulsification is an important unit operation in chemical engineering. Due to its high energy cost, there is substantial interest in increasing the fundamental understanding of drop breakup in these devices, e.g., for optimization. In this study, numerical breakup experiments are used to study turbulent fragmentation of viscous drops, under conditions similar to emulsification devices such as high-pressure homogenizers and rotor-stator mixers. The drop diameter was kept larger than the Kolmogorov length scale (i.e., turbulent inertial breakup). When varying the Weber number (We) and the disperse-to-continuous phase viscosity ratio in a range applicable to emulsification, three distinct breakup morphologies are identified: sheet breakup (large We and/or low viscosity ratio), thread breakup (intermediary We and viscosity ratio &gt; 5), and bulb breakup (low We). The number and size of resulting fragments differ between these three morphologies. Moreover, results also confirm previous findings showing drops with different We differing in how they attenuate the surrounding turbulent flow. This can create ‘exclaves’ in the phase space, i.e., narrow We-intervals, where drops with lower We break and drops with higher We do not (due to the latter attenuating the surrounding turbulence stresses more).

Population balance modelling of a continuous static mixer-based emulsification process

Emulsions preparation using static mixers is investigated in this work. First, it is demonstrated that a fully continuous emulsification process is experimentally feasible through the direct pumping of the two immiscible phases individually into a set of static mixers. This process is advantageous, especially in the case of highly viscous fluids, compared to that employing a pre-emulsification step in stirred tanks which creates inhomogeneity in the sample in terms of the droplet size. Second, a population balance model is employed to predict the droplet size distribution at the process outlet. To do so, a recently modified breakage kernel of Coulaloglou and Tavlarides, accounting for droplet breakage within the inertial and dissipation subranges of isotropic turbulence, is used. In this work, we propose to calculate the second-order longitudinal structure function using semi-empirical formulas. Moreover, the kernel considers the droplet cohesive force due to the viscosity of the dispersed. While this term is sometimes neglected in the literature, we found that this is certainly due to limited variation of the dispersed phase viscosity in the set used for parameter identification. The developed model is validated against a wide range of experimental data and showed very good prediction capabilities.

Temporal instability of confined three-dimensional liquid jet with heat and mass transfer under longitudinal acoustic oscillations

The temporal instability of a confined viscous liquid jet surrounded by high-speed co-flowing viscous gas phase is studied in this work. The effect of the longitudinal acoustic oscillations, which is regarded as gas axial velocity oscillations, is also considered. The heat and mass transfer is characterized by the ratio between conduction heat flux and the evaporation heat flux; then, an explicit dispersion relation equation is obtained. The results suggest that more than one unstable region appears because of the gas velocity oscillations, including Kelvin–Helmholtz (K–H) instability and parametric instability regions. Increasing the forcing frequency enhances the K–H instability, while it has a stabilizing effect on the parametric instability. In addition, the liquid jet tends to be more unstable in non-axisymmetric modes when the gas rotating strength is strong. Although the gas viscosity has a destabilizing effect on the gas–liquid interface, the destabilizing effect is weak due to the low viscosity of the gas phase. According to the linear instability theory, the dominant wavenumber will locate in the most unstable region. Moreover, the parametric instability in non-axisymmetric modes may be observable when the Weber number is large.

Slug bubble deformation and its influence on bubble breakup dynamics in microchannel

The deformation of moving slug bubbles and its influence on the bubble breakup dynamics in microchannel were studied. Three bubble morphologies were found in the experiment: slug, dumbbell and grenade shapes. The viscosity effect of continuous phase aggravates the velocity difference between the fluid near the wall and the bubble, resulting in that the continuous phase near the bubble head flows towards and squeezes the bubble tail, which causes the deformation of bubbles. Moreover, the experimental results show that the deformation of bubbles could significantly prolong the bubble breakup period at the downstream Y-junction. There exists the critical capillary number Ca Cr for the asymmetric breakup of grenade bubbles, Ca Cr increases with the rise of flow rate and viscosity of the continuous phase.

The effect of original and initial saturation on residual nonwetting phase capillary trapping efficiency

Injection of supercritical carbon dioxide (CO2) into geological formations is a strategy for both atmospheric greenhouse gas reduction (climate change mitigation) and enhanced oil recovery. To understand CO2 trapping efficiency, the capillary trapping behaviors that immobilize subsurface fluids were analyzed at the pore-scale using pairs of proxy fluids representing the range of in situ nonwetting and wetting fluid properties encountered in geologic storage reservoirs. The pairs of fluids were cycled through imbibition and drainage processes using a flow cell apparatus containing a sintered glass bead column. Computed x-ray microtomography (microCT) was used to identify immobilized nonwetting fluid after imbibition and drainage events. From microCT images, the trapped residual (post-secondary imbibition) nonwetting phase was spatially correlated to both the original (post-primary imbibition) and the initial (post-primary drainage) nonwetting phase; this relationship is referred to here as the original saturation dependence (SO-dependence) and initial saturation dependence (SI-dependence), respectively. Significant trends of decreasing SO- and SI-dependence with increasing wetting and nonwetting fluid phase viscosities were observed. This finding implies that the amount of CO2 injected and ultimately trapped is dependent on the nonwetting phase (e.g. oil or gas) already present in the formation, as well as on the manner in which supercritical CO2 is initially injected, which are factors not considered in current trapping models. To explore the potential effect that varying viscosity and interfacial tension (IFT) might have on this process, we report on a variety of fluid pairs with different viscosities and IFT.

Preparation of self-foamed glass ceramics based on the cooperative treatment of various solid wastes: Characterization of structure-properties and analysis of self-foaming behavior

Utilization of solid waste can be a preferable waste management solution. In this study, a type of self-foamed glass ceramic material was prepared using lead zinc mine tailings, red mud, and silica sand tailings through in-situ decomposition foaming. The effect of silica sand tailings content on the pore morphology, microstructure, and basic performances of the prepared specimen was analyzed. Meanwhile, the differences in the pore morphology and self-foaming behavior of the foamed glass ceramics were discussed. The obtained results indicated that the addition of silica sand tailings facilitated the formation of the glass phase in samples, and the composition, content, and viscosity of the glass phase could be changed by adjusting the content of silica sand tailings, resulting in a uniform distribution of the formed pores. Owing to their well-distributed pore structure, foamed glass ceramics exhibit excellent comprehensive properties (light weight and high strength), making them a potential non-load-bearing material. In this study, the comprehensive properties of the sample prepared with 40 wt% silica sand tailings were determined to be the best: exhibiting the lowest bulk density (0.63 g/cm3), highest porosity (72.6%), and an applicable mechanical strength (9.7 MPa). This study utilized three solid wastes through a reasonable combination based on their chemical composition, and proposed a novel technique for the preparation of self-foaming glass ceramics via the cooperative treatment of various wastes.

A fractal relative permeability model for two-phase flow through unsaturated shale porous medium

The accurate calculation of the two-phase relative permeability has a significant impact for effectively characterizing the fluid flow patterns of unsaturated shale reservoir. A new fractal relative permeability model is developed based on two-phase transport feature in confined nanopores, which is upscaled with the aid of fractal theory for two-phase flow through unsaturated shale porous medium. Unlike the earlier models, the presented models considered nanopore wettability, confined viscosity varies with the nanopore diameter (variable water phase viscosity), stress dependence effect, real gas effect, irreducible water saturation and tortuosity effect. The proposed model compares the permeability of single nanopore and multiple nanopores with earlier research, which shows that the fractal relative permeability model agrees well with earlier models and experimental data. The results show that the Monte Carlo model and Abaci experimental model studied by previous researchers are special cases of the proposed fractal model, thus showing that the proposed fractal model has obvious advantages. Further calculations show that 1) The gas phase’s relative permeability gradually decreases with the increase of water saturation; 2) Confined viscosity varies with the nanopore diameter has a greater influence on the inorganic pores and a smaller influence on the organic pores on the relative permeability; 3) The relative permeability of the intersection point in the organic pore is higher than that of the inorganic porous, but the water saturation at the intersection is less than that of inorganic pores. Therefore, it lays a solid foundation for revealing the two-phase flow law of shale porous media.

Homogenization improves foaming properties of insoluble pea proteins.

Foams are essential in many food applications and require surface-active ingredients such as proteins for formation and stabilization. We investigated the influence of high-pressure homogenization on foaming properties of insoluble pea protein dispersions (5% w/w) at pH 3 and 5. Unhomogenized insoluble pea protein dispersions did not foam at either pH 3 or 5, as they consisted of large insoluble pea protein aggregates with limited surface activity. At pH 3, the homogenized pea protein dispersions generated foams due to higher protein solubility and surface activity through disruption of large protein aggregates into smaller particles. The foam stability decreased with increasing homogenization pressure and number of cycles due to a reduction in continuous phase viscosity. At pH 5, the insoluble pea proteins foamed when the homogenization resulted in formation of aggregates made of smaller protein entities, which was the case for homogenization ≥ 100MPa and three cycles. In general, the foam capacity (amount of formed foam) was higher at pH 3 due to improved protein solubility and surface activity that facilitated incorporation of air, while the foam stability (resistance against foam collapse) was better at pH 5 because of the presence of larger protein aggregates that formed thicker and more viscous films around the air bubbles benefitting retention of gas bubbles. Overall, homogenization improved the foaming properties of insoluble pea proteins at pH 3 and 5. Practical Application Insoluble pea protein dispersions formed foams at pH 3 and 5 after homogenization highlighting the potential of this processing step for the food industry. The improvement in functionality of plant-derived ingredients helps to increase their use for consumer goods, thereby supporting the transition to more sustainable food system.

Taylor–Culick retractions and the influence of the surroundings

When a freely suspended liquid film ruptures, it retracts spontaneously under the action of surface tension. If the film is surrounded by air, the retraction velocity is known to approach the constant Taylor–Culick velocity. However, when surrounded by an external viscous medium, the dissipation within that medium dictates the magnitude of the retraction velocity. In the present work, we study the retraction of a liquid (water) film in a viscous oil ambient (two-phase Taylor–Culick retractions), and that sandwiched between air and a viscous oil (three-phase Taylor–Culick retractions). In the latter case, the experimentally measured retraction velocity is observed to have a weaker dependence on the viscosity of the oil phase as compared with the configuration where the water film is surrounded completely by oil. Numerical simulations indicate that this weaker dependence arises from the localization of viscous dissipation near the three-phase contact line. The speed of retraction only depends on the viscosity of the surrounding medium and not on that of the film. From the experiments and the numerical simulations, we reveal unprecedented regimes for the scaling of the Weber number${We}_{f}$of the film (based on its retraction velocity) or the capillary number${Ca}_{s}$of the surroundings versus the Ohnesorge number${Oh}_{s}$of the surroundings in the regime of large viscosity of the surroundings (${Oh}_{s} \gg 1$), namely${We}_{f} \sim {Oh}_{s}^{-2}$and${Ca}_{s} \sim {Oh}_{s}^{0}$for the two-phase Taylor–Culick configuration, and${We}_{f} \sim {Oh}_{s}^{-1}$and${Ca}_{s} \sim {Oh}_{s}^{1/2}$for the three-phase Taylor–Culick configuration.

Coalescence law of microdroplet swarms in microchannels

Droplet microfluidic technology refers to the technique of preparing and controlling microscale monodisperse droplets by shear force and interfacial tension of two immiscible phases accurately, which has a wide range of applications in biomedicine, chemical reaction enhancement, nanoparticle preparation, and other fields due to its efficient mass and heat transfer, easy manipulation, rapid response, and high throughput. In the application of droplet microfluidic technology, coalescence often occurs due to the structural complexity of microchannels, which limits the practical application performance. The mechanism of droplet coalescence has been widely studied in conventional reactors, among which the liquid film drainage theory is the most widely accepted. To achieve accurate control of the droplet coalescence process, there have been many studies on droplet coalescence in microchannels. A variety of microchannel structures have been proposed and the mechanism and influencing factors of double droplets coalescence have been explored. However, most of the existing studies on coalescence are based on cross structures, mainly studying the coalescence of double droplets in confined systems. There is still a lack of research on the coalescence process of droplet swarms, which is more widespread in practical applications. Therefore, this work used the rectangular expansion structure to realize the droplet coalescence and explored the effects of two-phase flow rate, continuous phase viscosity, surfactant concentration, and the size of expanding channel on the droplet coalescence behavior. The coalescence types, probabilities, and size distributions of microdroplet swarms under different factors were summarized. The time distribution of liquid film drainage in the droplet coalescence process and the influence of various factors on it were further analyzed based on the liquid film drainage theory.

Dripping and jetting generation mode in T-junction microchannels with contractive structures

Droplet generation in T-junction microchannels with contractive structures is investigated under different flow rates and viscosity conditions by numerical simulation to improve the monodispersity and controllability of the microdroplets. Basic flow modes are observed, including dripping, transition, and jetting, under various flow rates and dispersed phase viscosities. The flow condition for the transition mode is modeled as a function of capillary number, flow rate ratio, and viscosity ratio to indicate the conversion of generation modes. Unlike ordinary T-junction microchannels, in this case, the droplet diameter first decreases and then increases with the increasing viscosity ratio in T-junction microchannels with contractive structures. By analyzing the velocity fields, pressure fields, and forces in droplet formation, the dynamic mechanism of the viscosity ratio on the generation mode and droplet size is achieved. The droplet volume in the dripping mode is analyzed to propose a prediction formula that takes into account the influence of the viscosity ratio.